1. Field of the Invention
The present invention relates in general to differential-type voltage-controlled oscillators. In particular, the present invention relates to a differential-type voltage-controlled oscillator featuring low-frequency stability compensation.
2. Description of Related Art
Conventional voltage-controlled oscillators (hereafter referred to as VCO) employ an input voltage signal to control a bias current signal that is used as the basis for the generation of an electrical oscillation. The oscillation is generated at a frequency that is a function of the voltage of the input signal.
The purpose of constructing a VCO, as the name implies, is to generate an output signal having its frequency controlled by the voltage of the input signal. In other words, the voltage of the input signal is utilized as the input controlling parameter, and the desired result is that the frequency of the output signal, behaving as function of the input voltage, is governed by the voltage parameter on a one-to-one functional basis.
As is well known in the art, it is essential that a stable and known functional relationship should exist between the input voltage and the output frequency parameters. Without this functional characteristic, it is difficult to maintain precise control over the frequency of the generated signal. For example, it is well known to persons skilled in this art that temperature is an important factor that affects the precision of control over the voltage-frequency behavior of the conventional VCO.
Furthermore, inherent electronic noises that exist in digital electronic systems are another annoying factor that obstructs the precision control over the frequency generated by a conventional VCO. For example, noise signals that enter the VCO device via the junction pads of the power and ground lines create an interference that makes it difficult to attain the desired functionality of the VCO.
FIG. 1 (Prior Art) of the accompanying drawings of the present invention shows the schematic diagram of a conventional VCO, in which an input voltage signal V.sub.BVCO is input to the device to control the bias current I for control over the frequency of the generated output signal. In such a conventional arrangement, electronic noises that enter into the VCO device via the lead pads for the power and ground lines tend to produce at least the following disadvantages over the operation of the VCO: First, the duty cycle may not be maintained at 50% as a symmetric oscillation waveform, and second, phase jitters occur in the output frequency signal. In general, the conventional VCO design is characterized by poor noise immunity.
One proposed solution to the above-described poor noise immunity problem of the conventional VCO devices involves making a differential-type VCO utilizing the differential amplifier to improve the overall device noise immunity. FIG. 2 (Prior Art) of the drawing shows the schematic diagram of such a differential-type VCO device in which a voltage-to-current converter 50 is connected to a cascade of three consecutive differential amplifiers 10, 20 and 30 that are each accompanied by the respective one of the three N-type metal-oxide semiconductor (NMOS) transistors N.sub.5, N.sub.8 and N.sub.11. Clearly, the cascade is not limited to three, as can be appreciated by persons skilled in the art.
However, as is well known, a differential amplifier relies on the balanced matching among the dimensional factors of its constituent components to maintain its designed operation characteristics. In the implementation of the semiconductor integrated circuit devices, the differential amplifier characteristics are sensitive to change in its fabrication conditions. Any change in the conditions of fabrication is likely to produce a shift in the voltage-frequency characteristics of the produced VCO device.
These prior art differential-type VCO devices have several drawbacks. At least two of the issues are worth mentioning. First, the control of semiconductor fabrication conditions described above is difficult.
For example, consider again the differential-type VCO of FIG. 2 (Prior Art). In the first differential amplifier 10, the gate voltages of the PMOS load transistors P.sub.3 and P.sub.4 are provided by the PMOS transistor P.sub.2 of the voltage-to-current converter 50, while the load currents I.sub.3 and I.sub.4 of the PMOS transistors P.sub.3 and P.sub.4 respectively are determined by I.sub.5, which is the current passing through the bias current-providing NMOS transistor N.sub.5.
In an ideal situation when the channel width/length ratio (W/L ratio) of transistors P.sub.3 and P.sub.4 are equal, then: EQU (W/L).sub.P.sbsb.3 =(W/L).sub.P.sbsb.4
wherein W represents the transistor channel width and L represents the channel length, the following relationship is established ##EQU1## then, there will be a relationship between the currents: ##EQU2## and ##EQU3## then ##EQU4## therefore, EQU I.sub.5 =2(K.sub.1 .multidot.K.sub.2)I.sub.1 =2[K.sub.1 .multidot.K.sub.2 .multidot.Gm]Vin
wherein G.sub.m is the transconductance when the voltage V.sub.in is converted into current I.sub.1.
The input voltage V.sub.in is converted into current I.sub.1 by the voltage-to-current converter 50, with the condition that EQU I.sub.1 =Gm.multidot.Vin
Due to the fact that charging/discharging VCO devices have a oscillation frequency that is proportional to the value of the charging/discharging current, it is ensured that the prior art differential-type VCO device does establish an oscillation that has a frequency controlled by the value of the input voltage V.sub.in.
However, as indicated previously, the situation described by the equations outlined above is an ideal. In reality when these differential-type VCO devices are actually fabricated, the absolute symmetry in the dimensions of the transistor components is less than ideal. The following discussion outlines a situation when the distortion in the component dimensional symmetry is introduced into these prior art differential-type VCO devices. For example, when the width/length ratios of PMOS transistors P.sub.3 and P.sub.4 can still be considered to establish symmetry: ##EQU5## then, when all transistors P.sub.3, P.sub.4 and N.sub.5 are operating in their respective saturation regions and have the following current relationships: ##EQU6##
Thus, in the initial stage when I.sub.3 +I.sub.4 &gt;I.sub.5, the currents I.sub.3 and I.sub.4 would force the voltages V(A) and V(B) at nodes A and B respectively in the first differential amplifier 10 to rise. This rise of node voltage pushes PMOS transistors P.sub.3 and P.sub.4 from their saturation regions into their linear operating regions. The result is the continuous reduction in the currents I.sub.3 and I.sub.4 until the equilibrium condition of I.sub.3 +I.sub.4 =I.sub.5 is achieved. At the same time, the gain of the first differential amplifier 10 obtained from output signal V(A) and the input signal V(F) would be: ##EQU7## where g.sub.m is the transconductance and g.sub.d is the drain conductance, and A.sub.VVCO-1 is the voltage gain of the first differential amplifier that must be greater than 1 so that the VCO device can initiate its oscillation.
On the other hand, when the PMOS transistors P.sub.3 and P.sub.4 , as well as the NMOS transistors N.sub.3 and N.sub.4 are all operating in their respective saturation region, the following condition will be established: EQU (gm).sub.N.sbsb.4 =(gm).sub.N.sbsb.3 &gt;(gd).sub.P.sbsb.4 =(gd).sub.P.sbsb.3
When the PMOS transistors P.sub.3 and P.sub.4 are in their respective linear region of operation, the values (gd).sub.P.sbsb.3 and (gd).sub.P.sbsb.4 will increase, and if the width/length ratio W/L is relatively larger, then: ##EQU8##
Thus, the voltage gain A.sub.VVCO-1 might be smaller than the value of 1, if margins for the semiconductor fabrication process design for the PMOS transistor P.sub.3 and NMOS transistor N.sub.3 are not considered in advance. Such a prior art differential-type VCO with a voltage gain less than 1 will not oscillate at all, since all excitation in its circuitry will cease, with nothing but stable direct currents appearing on the circuit nodes.
In addition to the critical condition necessary for the prior art differential-type VCO devices to ensure the initiation of their oscillation, the second principle drawback of these prior art differential-type VCO devices concerns the oscillation stability in the low frequency region.
FIG. 3 (Prior Art) shows the voltage-frequency characteristics of a conventional differential-type VCO. As is depicted in the characteristics curve, the voltage-to-current converter 50 is itself operating in its non-linear region when the bias current in the device is relatively very small. The non-linearity of the voltage-frequency characteristics in this operating region results in the output frequency f with very high sensetivity with respect to the input voltage Vin. For example, the characteristic curve C.sub.1 become abruptly, when the input voltage Vin is V.sub.0 and the output frequency f is below f.sub.0. A typical phenomenon is then caused by the noise inherent in electrical circuit. A small amount of change in the input voltage V.sub.1 results in a drastic change in the output frequency f.sub.1. Therefore, there is poor noise immunity in the VCO device in that operating region. In the case of prior art differential-type VCO devices, the operating region having poor noise immunity is the low-frequency operating region. Of course, one can always set the effective VCO operating frequency range above this poor noise immunity frequency range, but this results in a severe restriction of the effective operating frequency range of the VCO.